Current solid state illuminated projectors producing more than about 1000 lumens utilize blue laser diodes and a spinning phosphor wheel. The illumination typically involves sequential generation of blue, green and red color light, and the sequentially different colored light is directed to a pixel light modulator of one type or other. The pixel light modulator may, for example, be a spatial light modulator (SLM) such as a Texas Instruments DLP® digital micromirror device (DMD) whose mirrors are individually set using pulse-width modulation (PWM) intensity grayscaling with settings synchronized to respective time segments for illumination of the mirrors by the respective sequentially generated colors.
In a typical solid state illumination (SSI) system, at least red, green and blue color illumination time segments are generated. Other color time segments (viz., yellow or other secondary color, white, and/or black periods) are also possible. In a usual arrangement, a green color light is generated indirectly by illuminating a green color-emitting segment of the spinning phosphor wheel with light from a typically blue laser light emitting diode (LED), while red color light is generated either indirectly by illuminating a red color-emitting phosphor segment of the same wheel with the blue laser light or directly by using a separate red light LED. The blue color light is usually generated directly using the blue laser light itself. A diffuser may be used to reduce speckle from the coherent light.
An example solid state projector using blue laser LEDs and a phosphor color wheel as an illumination system and a spatial light modulator for modulation of the generated illumination is described in application Ser. No. 13/652,129 filed Oct. 15, 2012, entitled “Projector Light Source and System, Including Configuration for Display of 3D Images,” the entirety of which is incorporated herein by reference. Such system utilizes blue lasers as a direct source of blue color light and utilizes the blue lasers as an indirect source of other color light by energizing other color light producing phosphors with the blue color light from the blue lasers. The overall layout for such a system is illustrated in FIG. 1 of Application No. 61/756,407 filed Jan. 24, 2013, entitled “Split Phosphor/Slit Color Wheel Segment for Color Generation in Solid-State Illumination System,” the entirety of which is incorporated herein by reference. Because the illumination system generates one output color directly from the input source light and one or more other output colors indirectly by secondary emission, projectors utilizing such illumination systems are often referred to as hybrid SSI laser projectors.
A typical phosphor color wheel as used in the described solid state projector has annular arcuate region segments (viz., sectors of an annulus defined by two radii separated by an inner angle and by the inner and outer arcs they intercept) coated with different color emitting phosphors disposed at respective angularly spaced positions in a ring, circumferentially about a circular wheel. An example phosphor color wheel of this type is shown in FIG. 2 of Application No. 61/756,407. For image display, the color wheel is rotated to move the phosphor coated ring through a given angular rotation (e.g., ½, 1 or 2 revolutions) during an image frame display time (eye integration time). The laser light input beam is directed onto the wheel annulus to illuminate an area (viz., spot) through which the different segments pass sequentially as the wheel rotates. The wheel is typically rotated at a constant rotational speed, with the angular extent of the respective different color generating segments determined, at least in part, by the relative brightnesses of the generated illumination.
In the example wheel shown in FIG. 2 of Application No. 61/756,407, the wheel is a circular wheel having a reflective aluminum front surface (surface facing the input beam) and different color generating annular sectors angularly spaced circumferentially about a marginal band. The wheel is mounted centrally for rotation about a shaft at an adjustably settable, constant rotational speed (viz., one revolution per image frame display time). The illustrated wheel has two instances of angularly-spaced blue, green, red and yellow color generating segments which are sequentially driven past the incident input beam. Although not required, the illustrated sequence is the same in each instance. The blue segment comprises a slit for generating blue color by passing the input beam through the slit and around a wraparound path (see FIG. 1) back to the projection optics. The green, red and yellow segments comprise respective annular regions coated with different color light-emitting phosphors for respectively emitting corresponding green, red and yellow color light when energized by the incident input beam.
When rotated at constant rotational speed, the arcuate (angular) extent of each segment determines the amount of time that the color generated by that segment will be available for modulation to produce the corresponding color intensity contribution for the various pixels of the displayed image. The relative arcuate extents are thus established, at least in part, based upon the relative maximum intensities of the segment generated colors. Thus, the blue segment (which generates the brightest color because it passes the input blue laser light directly for generation of the blue color generation) has the shortest angular extent, and green (which is the weakest intensity generated by incident laser light energization of the color producing phosphors) has the longest. The illustrative layout shown in FIG. 2 of Application No. 61/756,407, for example, provides blue, green, red and yellow color sequences using 2×28° blue laser light transmitting slit segments, 2×61° green light emitting phosphor segments, 2×51° red light emitting phosphor segments, and 2×40° yellow light emitting phosphor segments.
In such arrangement, phosphors determine the red and green color points, and laser light passing through the opening and the laser input beam wavelength determine the blue color point. For the wheel shown in FIG. 2 of Application No. 61/756,407, the opening defining the slit takes the form of a window with an arcuate metal strip left at the wheel circumference, radially outwardly bordering the window. This strip leaves the circular wheel with an unbroken outer edge that improves rotational stability and reduces audible noise generation. The laser beam (spot) is directed to completely pass through the window opening. To reduce speckle and otherwise smooth the directly utilized blue laser light, a diffuser may be added in the blue light source or reuse path.
Other arrangements for generating color sequences during relative movement of a color wheel and input light beam are also possible. An example color wheel having concentric annular tracks or rings of the respective different color emitting phosphors located at different radially spaced locations is described in Patent Application Pub. No. US 2011/0211333 A1, published Sep. 1, 2011, entitled “Wavelength Conversion,” the entirety of which is incorporated herein by reference.
An example of a blue laser light source used in SSI systems is a blue laser diode, such as commercially available from Nichia, that emits light in the 445-448 nm wavelength spectral region. Such laser diodes are relatively inexpensive and efficient. However, the dominant wavelengths of such less expensive laser diodes are shorter than typically used in non-SSI illumination system, so may result in a less aesthetically pleasing purplish blue color contribution in the displayed image.
Several approaches have been suggested to modify the blue color emitted using light from the blue lasers as a direct source for blue color generation. The use of cyan phosphor in combination with blue laser light in a system using a blue light wraparound path is described in Application No. 61/752,294 filed Jan. 14, 2013, entitled “Method of Utilizing a Cyan Phosphor to Color Correct a Laser Illuminated Projection System,” the entirety of which is incorporated herein by reference. The use of cyan phosphor in combination with blue laser light in a system using a blue light reflection path is described in Application No. 61/753,367 filed Jan. 16, 2013, entitled “Method of Utilizing a Cyan Phosphor to Color Correct a Laser Illuminated Projection System,” the entirety of which is incorporated herein by reference. Other approaches for combining phosphor emitted cyan light with blue laser light for blue color generation are described in Application No. 61/757,810 filed Jan. 29, 2013, the entirety of which is also incorporated herein by reference.